Abstract:
The quality of surface water is vital for maintaining sustainable agriculture,
particularly in regions that heavily depend on irrigation. The Walawe River Basin,
an essential hydrological system in Sri Lanka, spans multiple climatic zones and
supports extensive agricultural activities. However, there is a lack of research
concerning the spatial variation in irrigation water quality across these zones. This
study aims to examine surface water quality and its suitability for irrigation in the
intermediate, dry, and arid zones of the Walawe River basin during the dry season.
A total of 38 surface water samples were collected in March 2024. In situ
measurements included pH, electrical conductivity (EC), and turbidity. At the
same time, laboratory analyses determined the concentrations of anions and cations
utilizing various spectrophotometric techniques and Inductively Coupled Plasma
Optical Emission Spectroscopy (ICP-OES). To assess the suitability of the water
for irrigation, several established indices were calculated, including the Irrigation
Water Quality Index (IWQI), Sodium Adsorption Ratio (SAR), Per cent Sodium
(Na%), Kelly’s Ratio (KR), Permeability Index (PI), and Magnesium Hazard
(MH). Statistical differences across various climatic zones were evaluated using a
General Linear Model. The results revealed significant spatial variability across
several parameters, with the arid zone displaying higher pH values, electrical
conductivity (EC), alkalinity, chloride, fluoride, and sulfate levels (p < 0.05). This
variation is likely attributed to limited rainfall and increased evaporation. While
most sampling sites fell within acceptable limits for irrigation use, the elevated
sodium percentage (Na%) and lower permeability index (PI) values observed in
the dry and arid zones indicate potential long-term risks to soil permeability and
structure. The Irrigation Water Quality Index (IWQI) categorized all samples from
the intermediate and dry zones as highly suitable. In contrast, 10% of the samples
from the arid zone were downgraded to medium suitability. Overall, this study
highlights the impact of climatic variability on irrigation water quality within the
Walawe Basin. The findings advocate for implementing region-specific water
management practices and ongoing monitoring programs to support sustainable
agricultural development and mitigate potential future degradation of soil and
water resources.
